The present invention relates to a scanning probe microscope typified by a scanning atomic force microscope (AFM) and, more particularly, to a scanning probe microscope that provides high noise margin during high-magnification observations and offers observation images with good resolution.
In a scanning probe microscope such as an AFM, microscopic tissues and microstructures on a sample surface are detected by making use of interaction between the sample surface and a scanning probe. For this purpose, a cantilever comprising a cantilever having a probe tip at its front end is used as a scanning probe. Where such a cantilever is employed, when the probe is made to scan the sample surface, an attractive force or repulsive force based on an atomic force is produced between the sample surface and the probe. Accordingly, if this atomic force is detected as an amount of deflection of the cantilever, and if quite slight movement of the sample stage in the Z-direction is so controlled that this amount of deflection is kept constant, i.e., the gap between the sample surface and the probe is retained constant, then the used control signal or the detected amount of deflection itself is representative of the topography of the sample surface.
FIG. 6 is a block diagram showing one example of the structure of the prior art scanning probe microscope. A sample 52 is placed on a sample stage 55. A cantilever 53 has a free end to which a probe 54 is attached. This probe 54 is located above, and opposite to, the sample 52. The amount of deflection of the cantilever 53 is detected by measuring the spot position of laser light 72 emitted from a laser generator 71 and reflected by the back surface of the cantilever 53 with a position detector 73.
The position detector 73 is composed, for example, of four separate light-detecting electrodes. The detector is so aligned that when the amount of deflection of the cantilever 53 is zero, the spot of the laser light 72 is brought to the center of the 4 separate electrodes. Therefore, if the cantilever 53 deflects, the spot of the laser light 72 moves on the 4 separate electrodes. The output signals from the 4 electrodes produce a voltage difference. This voltage difference is amplified by a differential amplifier 74 and applied as a deflection amount signal S1 to the non-inverting input terminal (+) of a operational amplifier 75. A target value-setting portion 79 applies a target value signal regarding the amount of deflection of the cantilever 53 to the inverting input terminal (-) of the operational amplifier 75.
An error signal S2 delivered from the operational amplifier 75 is fed to a proportional-plus-integral-control portion (PI control portion) 76 via a low-pass filter 80. The PI control portion 76 combines the error signal S2 and its integral value and feeds the resulting signal as a PI control signal to a voice coil motor (VCM) driver amplifier 70 and to an observed image signal amplifier 77, the PI control signal acting also as an observed image signal. The VCM driver amplifier 70 produces a driving current, or an exciting current, corresponding to the voltage level of the PI control signal and excites a VCM 81. Thus, the cantilever 53 is made to move a slight distance corresponding to the voltage level of the PI control signal in the Z-direction. The observed image amplifier 77 amplifies the PI control signal by a factor that is specified by a magnification-setting portion 83 and corresponds to the magnification factor in the Z-direction. The amplified signal is supplied as an observed image signal S5 to an image display device (e.g., a CRT) 86.
A scanning signal-generating portion 78 produces XY scanning signals SX and SY to cause the cantilever 53 to move a slight distance in the X- and Y-directions. A magnification control portion 82 attenuates the scanning signals SX and SY by factors that correspond to the magnification factors in the X- and Y-directions, respectively, and are specified by the magnification-setting portion 83. The scanning signals SX and SY attenuated according to the magnification factors are supplied to a VCM driver amplifier 84, which in turn excites a VCM 85 according to the scanning signals SX and SY to drive the cantilever 53 over slight distances in the X- and Y-directions.
The observation magnification factors of the scanning probe microscope in the X- and Y-directions are enlarged by narrowing the range scanned by the probe across the sample surface. In the above-described prior art technique, the magnification control portion 82 attenuates the scanning signals SX and SY according to the magnification factors. The attenuated scanning signals are supplied to the VCM driver amplifier 84 via a scanning line L1, thus narrowing the range scanned.
In addition, with respect to the Z-direction, the scanning probe microscope permits the magnification factor to be set according to the state of the surface of the sample. With the aforementioned prior art technique, if the amplification factor of the PI control signal achieved by the observed image signal amplifier 77 is set high through the magnification-setting portion 83, extension is possible in the Z-direction.
FIG. 5 is a block diagram showing the structure of the prior art VCM driver amplifier 84 about the X- and Y-directions. The scanning signal attenuated by the magnification-setting portion 82 is applied to one differential input terminal of an operational amplifier A1 via the scanning line L1. A current that responds to the voltage level on the scanning line L1 and is amplified according to a reference voltage Vref applied to the other differential input terminal is produced from the output terminal of the operational amplifier A1 and fed to the VCM 85. The output current from the VCM 85 is furnished to a detecting resistor R. The voltage developed across the detecting resistor R is applied to one input terminal of an operational amplifier A2. The output voltage from the operational amplifier A2 is applied as the above-described reference voltage Vref to the other differential input terminal of the operational amplifier A1.
With the prior art technique described above, the voltage level of the scanning signal applied to the operational amplifier A1 decreases with increasing the magnification factor. Therefore, if noise of the same level is introduced in the scanning signal line L1, the ratio of the noise level to the scanning signal level increases with increasing the magnification factor, thus lowering the noise margin. Consequently, if slight noise is introduced in the scanning signal line L1, the resolution is greatly affected adversely.
On the other hand, with respect to the Z-direction, the voltage level of the PI control signal applied to the observed image signal amplifier 77 drops with reducing the unevenness of the sample surface. Therefore, if noise of the same level is introduced in the signal line, the ratio of the noise level to the level of the PI control signal increases with decreasing the unevenness of the sample surface. In consequence, the noise margin drops.
It is an object of the present invention to provide a scanning probe microscope that is free of the foregoing problems, always provides high noise margin irrespective of the magnification factor and the state of the surface of the sample, and enables observation with high resolution.